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Low Idling and Code P0240

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My 2002 Subaru outback has about 191000 miles on it and recently it has started to idle around 250-300 rpms. my boss told me to clean some kind of idler but idk where to find it or how to clean it. does anyone know?

Also my subaru keeps having the code P0240 - Catalyst System Efficiency below threshold (Bank 1), but we replaced the O2 sensor and the cat and the light keeps coming back on. Any suggestions on it?

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· Premium Member
01 VDC, 05 R Sedan, 06 BAJA EJ257
19,144 Posts
P0420 most times indicates an engine performance issue. Read through here:

I had a Dodge in today that had a 420 code and it was due to a deteriorated vacuum hose on the leak detection pump for the EVAP. I heard it whistling before I even scanned the ECM for error codes.

Both problems, the idle and 420 are probably related. It could be fuel delivery, clogged fuel filter, dirty air filter, carbon build up on the IAC (idle air control motor/valve), could be dirty throttle body causing irregular throttle position readings, clogged PCV valve or misfires/poor ignition timing.
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